(a) Field of the Invention
The present invention relates to active material for a positive electrode used in lithium secondary batteries, and more particularly to active material for a positive electrode used in lithium second batteries in which the active material has improved electrochemical characteristics and a capacity of the active material can be adjusted. The present invention also relates to a method of manufacturing the active material in which particles of the active material can be made uniformly and to minute sizes.
(b) Description of the Related Art
With the proliferation in the use of portable electronic devices in recent times, coupled with advancements made enabling increasingly smaller sizes and weights for these devices, research is being actively pursued to improve energy density capabilities of lithium secondary batteries.
Lithium secondary batteries utilize material that is able to undergo lithium ion intercalation and deintercalation respectively for a negative electrode and a positive electrode, and are filled with organic electrolyte or polymer electrolyte, which enable movement of lithium ions inside the battery (i.e., back to the negative electrode in the form of an ionic current). The lithium secondary battery generates electrical energy by processes of oxidation and reduction which take place when lithium ions undergo intercalation and deintercalation in the negative electrode and the positive electrode, respectively.
In the past, although lithium metal was used as the negative electrode active material in lithium secondary batteries, a serious problem of dendrite forming on a surface of the lithium metal resulted during charging and discharging. This may cause a short circuit, or more seriously may lead to the explosion of the battery. To prevent such problems, carbonaceous material is now widely used for the negative active material. Carbonaceous material is able to alternatingly either receive or supply lithium ions while maintaining its structural integrity and electrical properties, and half of a potential of the cell is identical to that of lithium metal during insertion and separation of ions.
For the active material of the positive electrode in secondary batteries, a metal chalcogenide compound, enabling insertion and separation of lithium ions, is generally used, i.e. composite metal oxides such as LiCoO.sub.2, LiMn.sub.2 0.sub.4, LiNiO.sub.2, LiNi.sub.1 x Co.sub.x O.sub.2 (0&lt;X&lt;1), and LiMnO.sub.2. Regarding the advantages and disadvantages of these different materials: the Mn-based active materials, LiMn.sub.2 O.sub.4, and LiMnO.sub.2, can easily synthesize, are less expensive than the other materials and give minimal negative affects on the environment, but capacities of these materials are low; LiCoO.sub.2 is widely used as it exhibits an electrical conductivity of roughly 10.sup.-2 to 1 S/cm at room temperature, provides a high level of battery voltage, and has exceptional electrode characteristics, but is unsafe when charging or discharging at a high rate, and is more costly than the other materials; and LiNiO.sub.2 has a high discharge and charge capacity and is the least expensive of the above active materials for the positive electrode, but does not synthesize easily.
Generally, such composite metal oxides are manufactured by mixing with a solid raw material powder, and this mixture undergoes a solid phase reaction for providing plasticity to the mixture. For example, Japanese Laid-open Publication No. Heisei 8-153513 (Sony Corp.) discloses a method for manufacturing LiNi.sub.1-x Co.sub.x O.sub.2 (0&lt;X&lt;1) in which after a hydroxide containing Ni(OH).sub.2 and Co(OH).sub.2 or Ni and Co is mixed and heat treated, the hydroxide is ground and fractionated to diameter sizes of the particles. In another method, LiOH, Ni oxide and Co oxide are reacted, and after undergoing a first sintering at 400 to 580.degree. C. to form an oxide, a second sintering is performed at 600 to 780.degree. C. to manufacture a perfect crystalline active material.
However, in the above conventional methods for manufacturing the composite metal oxides, a synthesis temperature is high; a particle size of the reaction material is large; and it is difficult to control physical properties such as particle shape, and surface and pore characteristics of the produced active material. Since the physical properties of such active materials greatly affect electrochemical characteristics of the battery, there is a need for a method enabling the physical properties of the electrode materials to be freely adjusted, thereby maximizing the characteristics of the battery.